Continuous processes for the highly selective conversion of sugars to propylene glycol or mixtures of propylene glycol and ethylene glycol

ABSTRACT

Continuous processes for making propylene glycol from ketose-yielding carbohydrates are disclosed which enhance the selectivity to propylene glycol.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

This is a continuation-in-part of U.S. application Ser. No. 15/612,700,filed Jun. 3, 2017, which claims the benefit of U.S. ProvisionalApplication No. 62/345,399, filed Jun. 3, 2016, each of which isincorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention pertains to processes for the production of propyleneglycol (1,2-propanediol), particularly high-efficiency, continuousprocesses for the conversion of sugars to propylene glycol or mixturesof propylene glycol and ethylene glycol.

BACKGROUND

Propylene glycol is a valuable commodity chemical that has a broad rangeof uses such as for antifreeze. Propylene glycol is currently made fromhydrocarbon feedstocks and by hydrogenolysis of glycerin.

Schreck, et al., in U.S. Pat. No. 9,399,610 B2, disclose improved,continuous processes for the conversion of carbohydrates to ethyleneglycol and propylene glycol. They disclose using a reactor for theconversion of carbohydrates to the glycols which has a first zonecomprising a retro-aldol catalyst and a second zone comprising aretro-aldol and reducing catalyst. Where the feed is an aldose,glycolaldehyde from the retro-aldol reaction is hydrogenated in thesecond zone of the reactor to ethylene glycol. They also disclose usingketose as the carbohydrate to produce propylene glycol.

Nevertheless, challenges still remain to further enhance the selectivityof the conversion of carbohydrates to glycols. These challenges are notinsignificant due to the myriad of reactions that can occur under theconditions required for the retro-aldol reaction and for thehydrogenation, including, but not limited to, hydrogenation of sugars tohexitol or pentitol (herein referred to as alkitols) and the formationof side products such as methane, methanol, ethanol, propanol, glycerin,1,2-butanediol, threitol, and humins. Although some side products may bemarketable, their recovery to meet merchant grade specification can becostly.

SUMMARY

By this invention, continuous processes are provided that enhance theselectivity of a retro-aldol and hydrogenation conversion ofcarbohydrate to propylene glycol or mixtures of propylene glycol andethylene glycol.

In accordance with this invention, a rapid heating of a carbohydratebeing fed into a reaction zone can reduce the production of alkitol andother side products. The mechanism by which the rapid heating of thecarbohydrate feed results in the enhance selectivity of conversion topropylene glycol or mixtures of ethylene glycol and propylene glycol isnot fully understood. Without wishing to be limited to theory, it isbelieved, in part, that at temperatures above about 225° C., preferablyabove 230° C., in the presence of retro-aldol catalyst, the rate of theretro-aldol conversion is sufficiently rapid that intermediates areconverted to propylene glycol or ethylene glycol, as the case may be, incomparison to side products.

Measuring directly the rate of heating of the carbohydrate feed isproblematic due to the speed at which the heating occurs. The problem oftemperature measurement is further confounded by the heat and masstransfer through the fluid which is being heated. The heat and masstransfer parameters within a given fluid will depend upon many factorsincluding, but not limited to, the method of heating, the temperaturedifferential, and the physical structure of the zone in which theheating is occurring. Moreover, the analytical techniques to measuretemperatures essentially in all regions of the fluid are practicallyunavailable. Accordingly, ascertaining whether or not the rate ofheating is sufficient can only practically be done by reference to therelative formation of certain compounds generated in the practice of theprocess. Nevertheless, it is believed that the rate of heating issufficient to raise the temperature of the entire carbohydrate feed fromabout 170° C. to at least 225° C., preferably to at least 230° C., inless than about 10 or 15 seconds, and more preferably in less than about5 seconds, and in some instances less than about 3 seconds, and in someother instances less than about 1 second, especially where isomerizationof the sugar, e.g., aldose to ketose or ketose to aldose, would beadverse to securing the desired conversion to propylene glycol ormixtures of propylene glycol and ethylene glycol having sought molecularratios. However, the processes of this invention also encompass theintentional isomerization of the carbohydrate feed, i.e., aldose toketose or ketose to aldose, to achieve a product having a desiredpropylene glycol to ethylene glycol mass ratios.

The temperature range through which the carbohydrate feed is to berapidly heated in accordance with processes of this invention is frombelow 170° C. to above 230° C. In some instances, the carbohydrate feedmay be at a temperature below about 150° C., or even below about 100°C., when the rapid heating commences. In some instances it is preferredthat where the carbohydrate feed contains retro-aldol catalyst, therapid heating of the carbohydrate feed commences prior to about 100° C.

In accordance with this invention, the continuous processes forconverting carbohydrate-containing feed, which feed contains at leastone of aldopentose and ketose-yielding carbohydrate, said ketosecomprising at least one of hexaketose and pentaketose, to propyleneglycol or a mixture of propylene glycol and ethylene glycol comprise:

-   -   a. continuously or intermittently passing said carbohydrate feed        into a reaction zone having an aqueous-hydrogenation medium        containing retro-aldol catalyst, hydrogen and hydrogenation        catalyst;    -   b. maintaining the aqueous-hydrogenation medium in the reaction        zone at hydrogenation conditions to provide a product solution        comprising propylene glycol and alkitol comprising at least one        of pentitol and hexitol, and optionally, ethylene glycol, said        hydrogenation conditions comprising a temperature in the range        of between about 230° C. to 300° C., a ratio of retro-aldol        catalyst to hydrogenation catalyst, and hydrogen partial        pressure that, in combination, are sufficient to convert at        least about 95 percent of the carbohydrate feed; and    -   c. continuously or intermittently withdrawing product solution        from said reaction zone,        wherein said carbohydrate feed is at least partially hydrated        and is under a pressure sufficient to maintain partial        hydration; wherein said carbohydrate feed is below a temperature        of about 170° C.; and wherein said carbohydrate feed is heated        to above 230° C. immediately prior to or in the reaction zone        and the rate of heating of the carbohydrate feed from below        170° C. to above 230° C. is sufficient to provide a product        solution having a mass ratio of total propylene glycol and        ethylene glycol to said alkitol greater than about 10:1,        preferably greater than about 20:1.

The carbohydrate feed as it contacts the aqueous hydrogenation mediumcontains at least one of aldopentose and a ketose-yielding carbohydrate.Aldopentose upon retro-aldol cleavage yields glycolaldehyde andglyceraldhyde. Glyceraldehyde can ultimately be converted to propyleneglycol. The ketose-yielding carbohydrate may be a ketose, per se, or maybe a di- or polysaccharide or hemicellulose that upon hydrolysis yieldsa ketose or a ketose precursor. Within the broad scope of thisinvention, the carbohydrate feed may also include an aldose-yieldingcarbohydrate or other aldose-yielding carbohydrate. It is understoodthat aldoses can isomerize to ketoses, and thus aldoses areketose-yielding carbohydrates. Conversely, ketoses can isomerize towardsequilibria with aldoses. The equilibria and kinetics of theisomerization reaction are affected by the presence and type ofisomerization catalyst, species of the aldose or ketose to beisomerized, the temperature, interaction with other components presentand other conditions of the feed as is well known in the art. In anoptional embodiment of the processes of this invention, the carbohydratefeed is maintained under isomerization conditions, which may becatalytic isomerization conditions, for a time sufficient to affect thedesired extent of isomerization. Generally, retro-aldol catalysts alsocatalyze isomerization. Hence, the isomerization can be, but need notbe, conducted in the presence of the retro-aldol catalyst. Often, thetemperature of the isomerization is below about 170° C., and preferablyin the range of about 100° C. to about 150° C. or 160° C.

In many instances, at least about 60 mass percent, and in someinstances, at least about 70 mass percent, of the total aldopentose,ketose-yielding and aldose-yielding, carbohydrate is converted topropylene glycol and ethylene glycol. The mass ratio of propylene glycolto ethylene glycol will depend upon the relative presence ofketose-yielding, aldopentose and aldose-yielding carbohydrates in thecarbohydrate feed and the extent of isomerization. Often, the mass ratioof propylene glycol to ethylene glycol is greater than about 1:5, say,greater than about 1:2, and sometimes greater than 1:1. Where thecarbohydrate-containing feed is at least about 50 mass percentketose-yielding carbohydrate, e.g., sucrose, the mass ratio of propyleneglycol to ethylene glycol is greater than about 1.5:1, say, greater thanabout 3:1. Often carbohydrate feeds that provide between about 120 and700 or 800, preferably between about 150 and 500, say, 200 to 400, gramstotal carbohydrate per liter of aqueous-hydrogenation medium, provideproduct solutions having advantageous ratios of propylene glycol toethylene glycol and reduced co-production of alkitols and1,2-butanediol.

In many instances the mass ratio of 1,2-butanediol in the product tototal propylene glycol and ethylene glycol is less than about 1:30, andmore preferably less than about 1:50.

The carbohydrate feed may be admixed with retro-aldol catalyst prior tobeing heated to a temperature at least 225° C., preferably at leastabout 230° C., or may be substantially devoid of any retro-aldolcatalyst. In some aspects of the invention, the carbohydrate feed willcontain retro-aldol catalyst. In these aspects, retro-aldol reactionsmay occur during the heating.

The heating of the carbohydrate feed can be effected by indirect heatexchange, direct heat exchange or a combination of both. In theembodiments of the processes of the invention where the carbohydratefeed is heated at least in part by direct heat exchange, the heating mayoccur immediately prior to or in the reaction zone. The warmer fluid forthe direct heat exchange may be any suitable fluid and often compriseswater. The temperature and amount of the warmer fluid are sufficient toenable the carbohydrate feed to achieve, in combination with any otherheating source, a temperature of at least 230° C. Often, the warmerfluid is above 230° C., and in some instances above 235° C. Thecarbohydrate feed can be introduced into a reaction zone for thehydrogenation, or, if used, a prior retro-aldol reaction zone having anessential absence of hydrogenation catalyst, and the aqueous mediumcontained in such reaction zone serves as the warmer fluid.Alternatively, a warmer fluid may be combined with the carbohydrate feedprior to the combination being introduced into a retro-aldol orcombination retro-aldol and hydrogenation reaction zone.

The commencement of the contact between the carbohydrate feed and theretro-aldol catalyst may occur in the reaction zone containing thehydrogenation catalyst or in a separate reaction zone. Where the contactis commenced in a separate reaction zone, all or a portion of thecarbohydrate can be reacted in the separate reaction zone. In someinstances, all or a portion of the ketose, and aldose, if present,undergoes retro-aldol conversion in the reaction zone containing thehydrogenation catalyst, e.g., at least about 10, and sometimes at leastabout 20, mass percent to essentially all of the ketose in thecarbohydrate feed undergoes retro-aldol conversion in the reaction zonecontaining the hydrogenation catalyst. In the certain embodiments of theinstantly disclosed processes, the retro-aldol catalyst is a homogeneouscatalyst and the hydrogenation catalyst is heterogeneous. Thus, thedispersion of the retro-aldol catalyst within the region occupied by thehydrogenation catalyst can provide intermediates that can providepropylene glycol proximate to hydrogenation sites.

The amount of hydrogenation catalyst required for a given circumstancewill depend upon the relative activity of the catalyst and the masstransfer of hydrogen and glycolaldehyde and intermediates to thecatalyst. A preferred hydrogenation catalyst is a supportednickel-containing hydrogenation catalyst, especially nickel catalystscontaining one or both of rhenium and iridium. The ratio of theretro-aldol catalyst to hydrogenation catalyst is preferably sufficientthat the production of alkitols, e.g., from the hydrogenation ofketose-yielding carbohydrate and aldose-yielding carbohydrate, isminimized. However, it is preferred that the hydrogenation catalyst havea density in the reaction zone sufficient to cause hydrogenation ofglycolaldehyde and other intermediates before competitive reactions ofintermediates are able to generate products other than propylene glycoland ethylene glycol.

In some instances, the carbohydrate feed can be a melted solid, in whichcase it should remain at least partially hydrated in order to avoidcaramelization during the heating. Preferably the carbohydrate feed isprovided as an aqueous solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a facility capable of using theprocess of this invention, according to certain embodiments.

DETAILED DESCRIPTION

All patents, published patent applications and articles referencedherein are hereby incorporated by reference in their entirety.

Definitions

As used herein, the following terms have the meanings set forth belowunless otherwise stated or clear from the context of their use.

Where ranges are used herein, the end points only of the ranges arestated so as to avoid having to set out at length and describe each andevery value included in the range. Any appropriate intermediate valueand range between the recited endpoints can be selected. By way ofexample, if a range of between 0.1 and 1.0 is recited, all intermediatevalues (e.g., 0.2, 0.3. 6.3, 0.815 and so forth) are included as are allintermediate ranges (e.g., 0.2-0.5, 0.54-0.913, and so forth).

The use of the terms “a” and “an” is intended to include one or more ofthe element described.

Admixing or admixed means the formation of a physical combination of twoor more elements which may have a uniform or non-uniform compositionthroughout and includes, but is not limited to, solid mixtures,solutions and suspensions.

Aldose means a monosaccharide that contains only a single aldehyde group(—CH═O) per molecule and having the generic chemical formulaC_(n)(H2O)_(n). Non-limiting examples of aldoses include aldohexose (allsix-carbon, aldehyde-containing sugars, including glucose, mannose,galactose, allose, altrose, idose, talose, and gulose); aldopentose (allfive-carbon aldehyde containing sugars, including xylose, lyxose,ribose, and arabinose); aldotetrose (all four-carbon, aldehydecontaining sugars, including erythrose and threose) and aldotriose (allthree-carbon aldehyde containing sugars, including glyceraldehyde).

Aldose-yielding carbohydrate means an aldose or a di- or polysaccharideor hemicellulose that can yield aldose upon hydrolysis. Most sugars arering structures under ambient conditions and thus the aldose form occursunder the conditions of the process of this invention. Sucrose, forexample, is an aldose-yielding carbohydrate even though it also yieldsketose upon hydrolysis.

Alkitol means pentitols and hexitols. Hexitol means a six carboncompound having the empirical formula of C₆H₁₄O₆ with one hydroxyl percarbon. Hexitols can have different stereoconfigurations, e.g., sorbitoland mannitol. Pentitol means a five carbon compound having the empiricalformula of C₅H₁₂O₅ with one hydroxyl per carbon. Pentitols can havedifferent stereoconfigurations. Although this definition is limiting topentitols and hexitols, it should be understood that the carbohydratefeed can contain one or more higher and lower carbon number -itols,including mono-, di- and tri-itols (“other -itols”). These other -itolsprovide additional carbohydrate for conversion to propylene glycoland/or ethylene glycol.

Aqueous and aqueous solution mean that water is present but does notrequire that water be the predominant component. For purposes ofillustration and not in limitation, a solution of 90 volume percent ofethylene glycol and 10 volume percent water would be an aqueoussolution. Aqueous solutions include liquid media containing dissolved ordispersed components such as, but not in limitation, colloidalsuspensions and slurries.

Bio-sourced carbohydrate feedstock means a product that includescarbohydrates sourced, derived or synthesized from, in whole or insignificant part, to biological products or renewable agriculturalmaterials (including, but not limited to, plant, animal, bacterial andmarine materials) or forestry materials.

Commencing contact means that a fluid starts a contact with a component,e.g., a medium containing a homogeneous or heterogeneous catalyst, butdoes not require that all molecules of that fluid contact the catalyst.

Compositions of aqueous solutions are determined using gaschromatography for lower boiling components, usually components having 3or fewer carbons and a normal boiling point less than about 300° C., andhigh performance liquid chromatography for higher boiling components,usually 3 or more carbons.

Conversion efficiency of a ketose to propylene glycol or aldohexose toethylene glycol is reported in mass percent and is calculated as themass of glycol contained in the product solution divided by the mass ofketose, or aldose as the case may be, theoretically provided by thecarbohydrate feed.

High shear mixing involves providing a fluid traveling at a differentvelocity relative to an adjacent area which can be achieved throughstationary or moving mechanical means to affect a shear to promotemixing. As used herein, the components being subjected to high shearmixing may be immiscible, partially immiscible or miscible.

Hydraulic distribution means the distribution of an aqueous solution ina vessel including contact with any catalyst contained therein.

Immediately prior to means no intervening unit operation requiring aresidence time of more than one minute exists.

Intermittently means from time to time and may be at regular orirregular time intervals.

Ketose means a monosaccharide containing one ketone group per molecule.Non-limiting examples of ketoses include ketohexose (all six-carbon,ketone-containing sugars, including fructose, psicose, sorbose, andtagatose), seduheptulose (seven-carbon ketone-containing sugar),ketopentose (all five-carbon ketone containing sugars, includingxylulose and ribulose), ketotetrose (all four-carbon, ketose containingsugars, including erythrulose), and ketotriose (all three-carbon ketosecontaining sugars, including dihydroxyacetone).

Ketose-yielding carbohydrate means a ketose or a di- or polysaccharideor hemicellulose that can yield ketose or ketose precursor uponhydrolysis. Most sugars are ring structures under ambient conditions andthus the ketose form occurs under the conditions of the process of thisinvention. Sucrose, for example, is a ketose-yielding carbohydrate eventhough it also yields aldose upon hydrolysis. For purposes herein,carbohydrates that produce both aldose and ketose will be deemedketose-yielding carbohydrate except as the context requires otherwise.

pH of an aqueous solution is determined at ambient pressure andtemperature. In determining the pH of, for example theaqueous-hydrogenation medium or the product solution, the liquid iscooled and allowed to reside at ambient pressure and temperature for 2hours before determination of the pH.

pH control agents means one or more of buffers and acids or bases.

A pressure sufficient to maintain at least partial hydration of acarbohydrate means that the pressure is sufficient to maintainsufficient water of hydration on the carbohydrate to retardcaramelization. At temperatures above the boiling point of water, thepressure is sufficient to enable the water of hydration to be retainedon the carbohydrate.

Rapid diffusional mixing is mixing where at least one of the two or morefluids to be mixed is finely divided to facilitate mass transfer to forma substantially uniform composition. Fractal mixing is rapid diffusionalmixing.

A reactor can be one or more vessels in series or in parallel and avessel can contain one or more zones. A reactor can be of any suitabledesign for continuous operation including, but not limited to, tanks andpipe or tubular reactor and can have, if desired, fluid mixingcapabilities. Types of reactors include, but are not limited to, laminarflow reactors, fixed bed reactors, slurry reactors, fluidized bedreactors, moving bed reactors, simulated moving bed reactors,trickle-bed reactors, bubble column and loop reactors.

Soluble means able to form a single liquid phase or to form a colloidalsuspension.

Carbohydrate Feed

The processes of this invention use a carbohydrate feed that contains aketose-yielding carbohydrate. In some instances, the carbohydrate feedcomprises at least about 40, and preferably at least about 50, masspercent of ketose-yielding carbohydrate based upon total carbohydrate inthe feed. Where product solutions containing a high mass ratio ofpropylene glycol to ethylene glycol are sought, the carbohydrate in thefeed comprises at least about 90, preferably at least about 95 or 99,mass percent of ketose-yielding carbohydrate. Often the carbohydratefeed comprises a carbohydrate polymer such as starch, cellulose, orpartially hydrolyzed fractions of such polymers or mixtures of thepolymers or mixtures of the polymers with partially hydrolyzedfractions. Preferred ketoses are ketohexoses and ketopentoses. Thecarbohydrate-containing feed can also contain other components includingthose that can be converted under the conditions of the processes ofthis invention to ethylene glycol or propylene glycol. Glycerin is anexample of a non-carbohydrate that can ultimately be converted topropylene glycol.

Most bio-sourced carbohydrate feedstocks yield glucose upon beinghydrolyzed. The processes of this invention can be effectively used forthe conversion of glucose and glucose precursors to propylene glycol andmixtures of propylene glycol and ethylene glycol. Ketose can be obtainedfrom carbohydrate polymers and oligomers such as hemicellulose,partially hydrolyzed forms of hemicellulose, disaccharides such assucrose, lactulose, lactose, turanose, maltulose, palatinose,gentiobiulose, melibiose, and melibiulose, or combinations thereof maybe used. However, the nature of these may result in variable mixtures ofethylene glycol and propylene glycol.

As stated above, the carbohydrate-containing feed can be subjected toisomerization conditions. An aldose-rich feed thus can be converted to afeed having a higher concentration of ketose, or a ketose-rich feed canbe converted to a feed having a higher concentration of aldose. In someinstances, the retro-aldol catalyst also promotes isomerization. Thus,if desired, the operator has the flexibility to use different feedstocksand to modulate the propylene glycol to ethylene glycol mole ratio. Whenisomerization is sought, it is typically effected at a temperature ofbetween about 100° C., or 120° C., to about 220° C. for between about 1and 120 minutes. Nevertheless, it is generally preferred to maintain theduration that the carbohydrate-containing feed is subjected totemperatures of 170° C. or higher to less than about 10 seconds. In anaspect of this invention, the isomerization is conducted underconditions that enable a desired mole ratio of propylene glycol toethylene glycol to be produced, which often are milder conditions, e.g.,temperatures in the range of about 120° C. to 170° C., and then the feedis rapidly heated to at least 225° C. or at least 230° C. to attenuatefurther isomerization.

The carbohydrate feed can be solid or in a liquid suspension ordissolved in a solvent such as water. Where the carbohydrate feed is ina non-aqueous environment, it is preferred that the carbohydrate is atleast partially hydrated. Most preferably, the carbohydrate feed isprovided in an aqueous solution. The mass ratio of water to carbohydratein the carbohydrate feed is preferably in the range of 4:1 to 1:4.Aqueous solutions of 600 or more grams per liter of certaincarbohydrates such as glucose and sucrose are sometimes commerciallyavailable. In some instances, recycle aqueous-hydrogenation solution oraliquot or separated portion thereof, may be contained in thecarbohydrate feed. Where the carbohydrate feed contains ethylene glycolor propylene glycol, the mass ratio of total ethylene glycol andpropylene glycol to carbohydrate is in the range of about 20:1 to 1:20.It is within the purview of this invention to add water to thecarbohydrate feed prior to introduction into the aqueous-hydrogenationmedium. The carbohydrate contained in the carbohydrate feed is providedin an amount of between about 120 to 700 or 800, often, about 150 to500, grams per liter of aqueous-hydrogenation medium. Optionally, aseparate reaction zone can be used that contains retro-aldol catalystwith an essential absence of hydrogenation catalyst. Where a separatereaction zone containing retro-aldol catalyst is used, it is preferredthat the carbohydrate contained in the carbohydrate feed to suchreaction zone provide between about 120 to 700 or 800, often, about 150and 500, grams of total carbohydrate per liter of aqueous medium in thatseparate zone.

Rapid Temperature Increase

In accordance with the first broad processes of this invention thecarbohydrate feed is rapidly transitioned through the temperature zoneof 170° C. to 225° C. or 230° C., and preferably to a temperature of atleast about 240° C. The rapid temperature increase attenuates the riskof caramelization of the carbohydrate or generation of products thatreduce the conversion to propylene glycol or the desired ratio of apropylene glycol to ethylene glycol product. In some instances, therapid heating through the temperature zone of 170° C. to 230° C. hasbeen found to provide a relatively low mass ratio of glycerin topropylene glycol. In such instances, the mass ratio of glycerin topropylene glycol is often less than about 0.5:1. The low production ofglycerin as a side product is advantageous due to the relatively lowmarket value of glycerin as compared to ethylene glycol and propyleneglycol.

The carbohydrate feed can be in the presence of other chemicals duringthe heating. For instance, hydrogen for the hydrogenation may be atleast in part supplied with the carbohydrate feed. Other adjuvants, suchas pH control agents, can also be present if desired. In one embodiment,the carbohydrate feed contains retro-aldol catalyst, and in suchinstances, catalytic conversion of the carbohydrate occurs during theheating. The extent of conversion of the carbohydrate during the heatingwill be affected, among other things, by the duration of the heating,the relative concentrations of the carbohydrate and the retro-aldolcatalyst and the activity of the retro-aldol catalyst.

As discussed above, the heating of the carbohydrate feed can beaccomplished in any suitable manner and one or more types of heating canbe used. All, none, or a portion of the heating of the carbohydrate feedcan occur before the carbohydrate feed is introduced into theaqueous-hydrogenation medium. For example, but not in limitation, theheating of the carbohydrate feed through the temperature zone of 170° C.to 225° C. or 230° C. can occur prior to introducing theaqueous-hydrogenation medium. In embodiments where the heatedcarbohydrate feed is maintained in contact with retro-aldol catalystprior to being introduced into the aqueous-hydrogenation medium, theduration of such contact prior to introduction into theaqueous-hydrogenation medium is generally below about 15, preferablybelow about 10, and in some instances below about 5, seconds. Typically,any hold time prior to the introduction of the heated carbohydrate feedinto the aqueous-hydrogenation medium is a consequence of the equipmentconfiguration such as piping distances and residence time in ancillaryequipment such as fluid distributors from the heat exchange zone intothe hydrogenation zone. As can be appreciated, turn up and turn downoperations will affect the inherent hold time.

The heat source used for the heating of the carbohydrate feed throughthe temperature zone of 170° C. to 225° C. or 230° C. is not critical.For instance, the heating can be provided by radiant or microwaveexcitation, indirect heat exchange with other process streams, or directheat exchange with a process stream also passing to theaqueous-hydrogenation medium or combinations thereof.

In instances where the carbohydrate feed through the temperature zone of170° C. to 225° C. or 230° C. is heated at least in part by direct heatexchange with the aqueous-hydrogenation medium, it is generallypreferred that retro-aldol catalyst is already present in theaqueous-hydrogenation medium. As discussed above, the rate of heatingwill be affected by heat and mass transfer parameters. It is generallydesired to promote mixing of the carbohydrate feed during the heating tofacilitate both mass and heat transfer and thereby reduce the timerequired for the carbohydrate feed to fully pass through thistemperature zone. This mixing can be affected in any suitable mannerincluding, but not limited to, mechanical and stationary mixing andrapid diffusional mixing. The thoroughness of the mixing also can affectthe mass transfer of reactants, intermediates, catalysts and productsand thus affect selectivities of conversion to propylene glycol andethylene glycol and the rate of formation of side products.

A particularly useful stream for direct heat exchange with thecarbohydrate feed is withdrawn product solution (recycle). If a solubleretro-aldol catalyst is used in the aqueous-hydrogenation medium therecycle provides for a substantial return of the retro-aldol catalyst tothe reaction system. The recycle can be at a temperature of at leastabout 180° C., say, at a temperature in the range of about 230° C. to300° C. The mass ratio of recycle to carbohydrate feed will depend uponthe relative temperatures of the two streams and the sought combinedtemperature. Often where a recycle is used, the mass ratio of recycle tocarbohydrate feed is in the range of about 1:1 to 100:1. The recycle maybe an aliquot portion of the withdrawn product solution, or may besubjected to unit operations to separate one or more components fromrecycle stream, such as, but not in limitation, degassing to removehydrogen and filtration to remove, e.g., any entrained heterogeneouscatalyst. Where the product solution is degassed to recover at least aportion of the hydrogen, the recycle is frequently an aliquot portion ofthe degassed product solution. One or more components can be added tothe recycle prior to combination with the carbohydrate feed in thedirect heat exchange in operation. These components include, but are notlimited to, retro-aldol catalyst, pH control agents, and hydrogen. Byusing a recycle of withdrawn product solution, the combined carbohydratefeed and recycle can contain unreacted aldose-yielding carbohydrate,intermediates to ethylene glycol, and ethylene glycol. Where acarbohydrate feed is used which is not in aqueous solution, e.g., is asolid or is a melt, the recycle provides water to dissolve thecarbohydrate and to stabilize the carbohydrate from caramelization.

The rate of heating, through kinetic influences, can affect the degreeof isomerization towards equilibrium of the aldoses or ketoses in thecarbohydrate feed and thus affect the ratio of propylene glycol toethylene glycol in the product. The slower the rate of heating, the moretime is available at elevated temperature for the carbohydrate to bedriven toward equilibrium. While higher temperatures generally increasethe rate of approach to equilibrium, the equilibrium itself can beshifted. Thus the operator needs to factor in the relative durations ofthe carbohydrate feed at each temperature. For instance, using a ketoseas the carbohydrate feed, where a higher ethylene glycol to propyleneglycol content in the produce solution is sought, a longer residencetime may be desired at lower temperatures where the equilibrium is at ahigher concentration of aldose. And then, the carbohydrate feed is veryrapidly heated and subjected to hydrogenation conditions to prevent theundue reversion of aldose to ketose which would otherwise occur at theequilibria at higher temperatures that have a lower concentration ofaldose. The presence of isomerization catalyst enhances the rate thatthe feed proceeds towards equilibrium. Retro-aldol catalysts typicallyact as isomerization catalysts. Thus, the carbohydrate-containing feedmay or may not contain retro-aldol catalyst depending upon the desireddegree of isomerization during the rapid heating. Other isomerizationcatalysts can be used in combination with or instead of the retro-aldolcatalyst. Chemical catalysts for isomerization include homogeneous orheterogeneous acids or bases as are well known. Enzymatic processes arealso well known to isomerize carbohydrates; however, these processesmust be operated at temperatures below those that adversely affect theenzyme. Hence, for thermophilic enzymes, preferred temperatures arebelow about 80° C. or 85° C.

The Conversion Process

In the processes of this invention, the carbohydrate feed is introducedinto an aqueous-hydrogenation medium that contains retro-aldol catalyst,hydrogen and hydrogenation catalyst. The carbohydrate feed may or maynot have been subjected to retro-aldol conditions prior to beingintroduced into the aqueous-hydrogenation medium, and the carbohydratefeed may or may not have been heated through the temperature zone of170° C. to 225° C. or 230° C. upon contacting the aqueous-hydrogenationmedium. Thus, in some instances the retro-aldol reactions may not occuruntil the carbohydrate feed is introduced into the aqueous-hydrogenationmedium, and in other instances, the retro-aldol reactions may have atleast partially occurred prior to the introduction of the carbohydratefeed into the aqueous-hydrogenation medium. It is generally preferred toquickly disperse the carbohydrate feed in the aqueous-hydrogenationmedium especially where the aqueous-hydrogenation medium is used toprovide direct heat exchange to the carbohydrate feed. This dispersioncan be achieved by any suitable procedure including, but not limited to,the use of mechanical and stationary mixers and rapid diffusionalmixing.

The preferred temperatures for retro-aldol reactions are typicallybetween about 225° C. or 230° C. and 300° C., and more preferablybetween about 240° C. and 280° C. The pressures (gauge) are typically inthe range of about 15 to 200 bar (1500 to 20,000 kPa), say, betweenabout 25 and 150 bar (2500 and 15000 kPa). Retro-aldol reactionconditions include the presence of retro-aldol catalyst. A retro-aldolcatalyst is a catalyst that catalyzes the retro-aldol reaction. Examplesof compounds that can provide retro-aldol catalyst include, but are notlimited to, heterogeneous and homogeneous catalysts, including catalystsupported on a carrier, comprising tungsten and its oxides, sulfates,phosphides, nitrides, carbides, halides and the like. Tungsten carbide,soluble phosphotungstens, tungsten oxides supported on zirconia, aluminaand alumina-silica are also included. Preferred catalysts are providedby soluble tungsten compounds, such as ammonium metatungstate. Otherforms of soluble tungstates, such as ammonium paratungstate, partiallyneutralized tungstic acid and sodium metatungstate. Without wishing tobe limited to theory, the species that exhibit the catalytic activitymay or may not be the same as the soluble tungsten compounds introducedas a catalyst. Rather, a catalytically active species may be formed inthe course of the retro-aldol reaction. The concentration of retro-aldolcatalyst used may vary widely and will depend upon the activity of thecatalyst and the other conditions of the retro-aldol reaction such asacidity, temperature and concentrations of carbohydrate. Typically, theretro-aldol catalyst is provided in an amount to provide between about0.05 and 100, say, between about 0.1 and 50, grams of tungstencalculated as the elemental metal per liter of aqueous-hydrogenationmedium. The retro-aldol catalyst can be added as a mixture with thecarbohydrate feed or as a separate feed to the aqueous-hydrogenationmedium or both.

Where the carbohydrate feed is subjected to retro-aldol conditions priorto being introduced into the aqueous-hydrogenation medium, preferablythe introduction into the aqueous-hydrogenation medium occurs in lessthan one, sometimes less than about 0.5, and in some instances less thanabout 0.1, minute from the commencement of subjecting the carbohydratefeed to the retro-aldol conditions. Often, at least about 10, preferablyat least about 20, percent of the aldose-yielding carbohydrate in thecarbohydrate feed is remaining upon introduction into theaqueous-hydrogenation medium. By continuing the retro-aldol conversionsof the carbohydrate in the aqueous-hydrogenation medium, the duration oftime between the retro-aldol conversion of the ketose or aldopentose tothe commencement of contact with the hydrogenation catalyst is reduced.

Typically the aqueous-hydrogenation medium is maintained at atemperature of at least about 225° C. or 230° C. until at least about95, preferably at least about 98, mass percent, or in some instancessubstantially all, aldopentose, ketose-yielding and aldose-yieldingcarbohydrates are reacted. Thereafter, if desired, the temperature ofthe aqueous-hydrogenation medium can be reduced. However, thehydrogenation proceeds rapidly at these higher temperatures. Thus thetemperatures for hydrogenation reactions are frequently between about225° or 230° C. and 300° C., say, between about 235° or 240° C. and 280°C. The pressures (gauge) are typically in the range of about 15 to 200bar (1500 to 20,000 kPa), say, between about 25 and 150 bar (2500 to15,000 kPa). The hydrogenation reactions require the presence ofhydrogen as well as hydrogenation catalyst. Due to the low solubility ofhydrogen in aqueous solutions, the concentration of hydrogen in theaqueous-hydrogenation medium will primarily be determined by the partialpressure of hydrogen in the reactor. The pH of the aqueous-hydrogenationmedium is often at least about 3, say, between about 3.5 and 8, and insome instances between about 4 and 7. Adjuvants that have the effect ofmodulating the pH may also be used.

The hydrogenation is conducted in the presence of a hydrogenationcatalyst. The hydrogenation catalyst can also be referred to as reducingmetal catalysts and are catalysts for the reduction of carbonyls.Frequently the hydrogenation catalyst is a heterogeneous catalyst. Itcan be deployed in any suitable manner, including, but not limited to,fixed bed, fluidized bed, trickle bed, moving bed, slurry bed, andstructured bed. Nickel, palladium and platinum are among the more widelyused reducing metal catalysts. However many reducing catalysts will workin this application. The reducing catalyst can be chosen from a widevariety of supported transition metal catalysts. Nickel, Pt, Pd andruthenium as the primary reducing metal components are well known fortheir ability to reduce carbonyls. One particularly favored catalyst forthe reducing catalyst in this process is a Ni—Re catalyst supported onsilica alumina. A similar version of Ni/Re or Ni/Ir can be used withgood selectivity for the conversion of the formed glycolaldehyde toethylene glycol. Nickel-rhenium is a preferred reducing metal catalystand may be supported on alumina-silica, silica or other supports.Supported Ni—Re catalysts with B as a promoter are useful. The reducingcatalyst may be pre-treated or treated during the hydrogenation tomoderate or passivate unduly active catalyst sites. The treatment maycomprise one or more of limited sintering, coking or contact with agentssuch as molybdenum, tungsten and rhenium that can passivate or occludeactive sites. When the treatment is conducted during the hydrogenationprocess, soluble salts of these agents can be added initially,continuously or intermittently. The adjuvants, in some instances, canassist in maintaining the retro-aldol catalyst in an active form.

Frequently in a slurry reactor, the hydrogenation catalyst is providedin an amount of between about 0.1 to 100, and more often, between about0.5 or 1 and 50, grams per liter of aqueous-hydrogenation medium and ina packed bed reactor the hydrogenation catalyst comprises about 20 to 80volume percent of the reactor.

Typically the retro-aldol reaction proceeds more quickly than thehydrogenation reaction and consequently the residence time of thecarbohydrate feed in the hydrogenation reactor is selected to reflectthe sought degree of hydrogenation. In some instances, the weight hourlyspace velocity is between about 0.01 and 20, and frequently betweenabout 0.02 and 5, hr⁻¹ based upon total carbohydrate in the feed. Insome instances it is desired to maintain the aqueous-hydrogenationmedium as well dispersed to assure relatively uniform concentrations ofintermediates to ethylene glycol therein.

The retro-aldol and hydrogenation environment can result in undesiredreactions. See, for instance, Green Chem., 2014, 16, 695-707. Table 1 onpage 697 and Table 4 on page 700 of the article reports the productcomposition from subjecting various aldoses to retro-aldol andhydrogenation conditions. The primary side products they report includesorbitol, erythritol, propylene glycol and glycerol.

In the processes of this invention, the combination of reactionconditions (e.g., temperature, hydrogen partial pressure, concentrationof catalysts, hydraulic distribution, and residence time) are sufficientto convert at least about 95, often at least about 98, mass percent andsometimes essentially all of the aldopentose, ketose-yielding andaldohexose-yielding carbohydrate in the feed. It is well within theskill of the artisan having the benefit of the disclosure herein todetermine the set or sets of conditions that will provide the soughtconversion of the carbohydrate.

Without wishing to be limited by theory, it is believed that theformation of intermediates by the retro-aldol reaction needs to be inclose time proximity to the hydrogenation of those intermediates topropylene glycol such that they are hydrogenated before a significantamount of the intermediates can be consumed in competitive reactions.Accordingly, a balance between the retro-aldol catalyst andhydrogenation catalyst can be ascertained for a given retro-aldolcatalyst and a given hydrogenation catalyst under the conditions of thereaction in order to achieve a high conversion efficiency to propyleneglycol and mixtures of propylene glycol and ethylene glycol.Additionally it is believed that the rapid heating of the carbohydratefeed provides the feed at temperatures where the retro-aldol rate ofreaction can be more easily matched to the hydrogenation rate ofreaction.

It is believed that the ratio of the retro-aldol catalyst tohydrogenation catalyst can also serve to attenuate the production ofalkitol by both minimizing the presence of carbohydrate and providingconcentrations of intermediates to propylene glycol and ethylene glycolthat preferentially go to the active hydrogenation sites. One mode ofoperation of processes in accordance to certain embodiments uses ahomogeneous retro-aldol catalyst and a heterogeneous hydrogenationcatalyst such that retro-aldol catalyst can be physically locatedproximate to the hydrogenation catalyst. The intermediates, beingsmaller molecules, diffuse more rapidly to the catalyst sites than thelarger carbohydrate molecules, and with the limited solubility ofhydrogen in the aqueous-hydrogenation medium, hydrogen mass transferrates to the hydrogenation catalyst are believed to modulate thehydrogenation reaction. Preferably the mass ratio of total propyleneglycol and ethylene glycol to alkitol in the product solution is greaterthan about 10:1, and in some instances is greater than about 20:1 or25:1 or even greater than about 40:1 or 50:1. As discussed above,providing total carbohydrate in the carbohydrate feed in an amount ofabout 120 to 700 or 800, or 150 to 500, grams per liter ofaqueous-hydrogenation medium can serve to attenuate the rate ofproduction of 1,2-butanediol.

Determining a suitable ratio of retro-aldol catalyst to hydrogenationcatalyst is within the skill of the artisan having the benefit of thedisclosures herein. The ratio will depend on, among other things, therelative activities of the two catalysts under steady-state conditions.The relative activities are influenced by the intrinsic activity of thecatalysts, and the physical configuration of the catalysts. Hence, theratios of these catalysts can vary widely over a range of retro-aldolcatalysts and hydrogenation catalysts. However, for a given retro-aldolcatalyst and hydrogenation catalyst, desirable ratios can be determined.If a retro-aldol reaction zone having an essential absence ofhydrogenation catalyst is used, as taught by Schreck, et al., in U.S.Pat. No. 9,399,610 B2, the conditions, including, but not limited to,hydraulic residence time and retro-aldol catalyst concentration, can beadjusted to achieve the sought conversion efficiencies. If desired, thereaction zone containing the hydrogenation catalyst can have differingratios of retro-aldol catalyst to hydrogenation catalyst. For example,in a continuous, stirred tank reactor using a homogeneous retro-aldolcatalyst and a heterogeneous hydrogenation catalyst and the carbohydratefeed being introduced at or immediately below the surface of theaqueous-hydrogenation medium, the rate of stirring may be such that adensity gradient for the hydrogenation catalyst exists. The lesserconcentration of hydrogenation catalyst at the top of theaqueous-hydrogenation medium enables carbohydrates to be subjected tothe retro-aldol reaction prior to significant amounts of hydrogenationoccurring.

Post Reaction Processing

A product solution is withdrawn from the reaction zone continuously orintermittently. Following the reactor, a part of the withdrawn productsolution may be separated for recycle back to the front of the processas described above. Preferably, at least a portion of the retro-aldolcatalyst is recycled or recovered from the withdrawn product solutionfor recycle. The withdrawn product solution can be depressurized withthe gases being captured for recovery of the hydrogen and removal ofunwanted gaseous by-products such as methane and carbon dioxide.

Upon cooling, less soluble portions of catalysts that are solubilizedfrom the bed or that are fed to the reactor are removed at the reducedtemperature and the remaining liquid is transferred to the recoveryportion of the process. Depending upon catalyst stability andsolubility, it is possible to take the degassed reactor effluent torecovery where a portion of the volatile products are recovered and theheavy bottoms are treated for, e.g., recovery of the tungsten catalystfor reuse in the reactor.

In recovery the low boiling components such as ethanol and methanol areremoved via distillation. Water is also removed via distillationfollowed by recovery of propylene glycol and ethylene glycol.

It is likely that separation of the ethylene glycol from the propyleneglycol or other close boiling glycols will require an additional, moresophisticated separation technology. Simulated moving bed technology isone such option that can be used. The choice is dependent on the qualityof the product that is required by the desired end use for the product.

Drawing

Reference is made to the drawing which is provided to facilitate theunderstanding invention but is not intended to be in limitation of theinvention. The drawing is a schematic depiction of an apparatusgenerally designated as 100 suitable for practicing the instantlydisclosed processes. The drawing omits minor equipment such as pumps,compressors, valves, instruments and other devices the placement ofwhich and operation thereof are well known to those practiced inchemical engineering. The drawing also omits ancillary unit operations.

A carbohydrate feed is provided by line 102. The carbohydrate feed maybe a solid or liquid including in a solution with water. For purposes ofdiscussion, the carbohydrate feed is an aqueous sucrose solution. Aretro-aldol catalyst is provided via line 104. The addition of aretro-aldol catalyst is optional at this point in the process. Forpurposes of discussion, the retro-aldol catalyst is ammoniummetatungstate in an aqueous solution, and the ammonium metatungstate isprovided in an amount sufficient to have a concentration of ammoniummetatungstate of about 10 grams per liter.

The carbohydrate feed is then combined with a warmer, recycle stream ofwithdrawn product solution as will be described later. This combinationaffects a direct heat exchange to increase the temperature of thecarbohydrate feed and provide a combined stream. This combined streamthen passes via line 106 to distributor 108 and reactor 110. Distributor108 may be of any suitable design. For purposes of discussion,distributor 108 is spray head that distributes the combined stream asfine droplets over the surface of the aqueous-hydrogenation medium 112in reactor 110. Reactor 110 contains agitator 114 to provide mechanicalmixing of the aqueous-hydrogenation medium 112. This mechanical mixingassist in dispersing the fine droplets of the combined stream within theaqueous-hydrogenation medium to further enhance the rate that thecombined stream is brought to the temperature of theaqueous-hydrogenation medium. It also assists in the mass transfer ofintermediates from the retro-aldol reaction to the hydrogenationcatalyst. Reactor 110 also contains a particulate, heterogeneoushydrogenation catalyst, by way of example, nickel/rhenium/boronhydrogenation catalyst on silica support, which catalyst is dispersed inthe aqueous-hydrogenation medium by the mechanical mixing.

Hydrogen is supplied to reactor 110 through line 116. The hydrogen maybe supplied through a nozzle to provide small bubbles of hydrogen tofacilitate mass transfer of hydrogen into the aqueous-hydrogenationmedium. If desired, an additional retro-aldol catalyst and otheradjuvants may be supplied to the reactor the line 118.

Aqueous-hydrogenation medium is withdrawn from reactor 110 via line 120as the product solution. As shown, a portion of the product solution ispassed via line 122 to line 106 as recycle to be combined with thecarbohydrate feed 102. This recycle will contain homogeneous retro-aldolcatalyst. Optionally, the recycle stream in line 122 can be heated inindirect heat exchanger 124 to enable the combined stream in line 106 tohave a higher temperature.

The following examples are provided to further illustrate the inventionand are not in limitation of the invention. All parts and percentagesare by mass unless otherwise stated. The following general procedure isused in Examples 1 and 2.

A 300 ml Hastelloy C Parr reactor is equipped with an agitator and oneor two feed supply lines and a dip tube attached to a sample bomb. Theend of the dip tube is positioned such that about 100 milliliters ofsolution would remain in the reactor. The reactor is charged withheterogeneous hydrogenation catalyst and an aqueous solution of ammoniumtungstate (1.0 mass percent). The charge is approximately 170milliliters of aqueous solution. The reactor is then sealed and purgedto remove oxygen. Purging is accomplished by three cycles ofpressurizing the reactor to 50 psig (345 kPa gauge) with nitrogen, thenventing to atmospheric pressure. The liquid level in the reactor isreduced to about 100 milliliters by draining through the dip tube. Anadditional three cycles of purging, while the aqueous solution is beingstirred, are conducted using hydrogen to reduce the concentration ofnitrogen, then venting to atmospheric pressure.

Stirring is commenced and is at a rate sufficient to maintain theheterogeneous hydrogenation catalyst in a slurry dispersion. The reactoris heated to 245° C. and pressurized under hydrogen to 10700 kilopascalsgauge. When the reactor reaches operating temperature and pressure, feedof sucrose solution is initiated and maintained at a constant 1.0milliliter per minute for the duration of the run. Retro-aldol catalystis continuously added at a constant rate of 1 mass percent per liter forthe duration of the run. Near-continuous operation is achieved bydraining the reactor at regular intervals to a constant liquid leveldetermined by the position of the dip tube. A filter attached to the endof the dip tube ensures all heterogeneous catalyst particles areretained inside the reactor. Every 10 to 15 minutes, the reactorpressure is adjusted to 10700 kilopascals by either adding hydrogen orventing.

A sample of the aqueous medium is taken during operation through the diptube and sample bomb upon reaching four hours of operation and cooled toroom temperature. The sample is analyzed by high pressure liquidchromatography (HPLC) and gas chromatography (GC). The HPLC is equippedwith a refractive index detector and uses a Hi-Plex H resin columnavailable from Agilent Technologies, Santa Clara, Calif. The GC analysisis performed with an HP 5890 GC (Agilent Technologies, Santa Clara,Calif.) using a flame ionization detector with 25:1 split injection. AJ&W DB-WAX 30 m×0.32 mm×0.5 micron capillary column (AgilentTechnologies, Santa Clara, Calif.) is used.

The hydrogenation catalyst is a nickel, rhenium and boron on silicaalumina catalyst prepared using the procedure set forth at column 8,line 62, to column 9, line 27, of U.S. Pat. No. 6,534,441. The silicaalumina support is a 3 millimeter extrudate and has a surface area ofabout 125 square meters per gram and a pore volume of about 0.7 to 0.9milliliters per gram. The catalysts contain about 6.8 mass percentnickel, unless otherwise stated, and the mass ratio of the atoms ofnickel:rhenium:boron is about 5:1.3:1.

Example 1

In this example, sucrose is used as the carbohydrate feed and isprovided at a concentration of about 32.4 mass percent in an aqueoussolution except as otherwise noted. The sucrose-containing feed containsretro-aldol catalyst. The supply line that is used has a 1/16 inch (1.6millimeter) outside diameter and length of 5 centimeters. At a feed rateof 1 milliliter per minute, the residence time per 2.5 centimeters oflength is about 0.4 second for a 1/16 inch supply line.

The sample contains propylene glycol and ethylene glycol in a mass ratiogreater than about 3:1; a ratio of total propylene glycol and ethyleneglycol to 1,2-butanediol greater than about 20:1; and a ratio of totalpropylene glycol and ethylene glycol to alkitol greater than about 10:1.

Example 2

In this example, glucose is used as the carbohydrate feed and isprovided at a concentration of about 32.4 mass percent in an aqueoussolution except as otherwise noted. The glucose-containing feedcontains, when a single feed supply line is use, retro-aldol catalyst.The supply line that is used has a 1/16 inch (1.6 millimeter) outsidediameter and length of 20 centimeters. At a feed rate of 1 milliliterper minute, the residence time per 2.5 centimeters of length is about0.4 second for a 1/16 inch supply line.

The sample contains propylene glycol and ethylene glycol in a mass ratiogreater than about 1:2; a ratio of total propylene glycol and ethyleneglycol to 1,2-butanediol greater than about 20:1; and a ratio of totalpropylene glycol and ethylene glycol to alkitol greater than about 10:1.

It is claimed:
 1. A continuous process for convertingcarbohydrate-containing feed, which feed contains at least one ofaldopentose and ketose-yielding carbohydrate, said ketose comprising atleast one of hexaketose and pentaketose, to propylene glycol or amixture of propylene glycol and ethylene glycol comprising: a.continuously or intermittently passing said carbohydrate feed into areaction zone having an aqueous-hydrogenation medium containingretro-aldol catalyst, hydrogen and hydrogenation catalyst; b.maintaining the aqueous-hydrogenation medium in the reaction zone athydrogenation conditions to provide a product solution comprisingpropylene glycol and alkitol comprising at least one of pentitol andhexitol, and optionally, ethylene glycol, said hydrogenation conditionscomprising a temperature in the range of between about 225° C. to 300°C., a ratio of retro-aldol catalyst to hydrogenation catalyst, andhydrogen partial pressure that, in combination, are sufficient toconvert at least about 95 percent of the carbohydrate; and c.continuously or intermittently withdrawing product solution from saidreaction zone, wherein said carbohydrate feed is at least partiallyhydrated and is under a pressure sufficient to maintain partialhydration; wherein said carbohydrate feed is below a temperature ofabout 170° C.; and wherein said carbohydrate feed is heated to above225° C. immediately prior to or in the reaction zone and the rate ofheating of the carbohydrate feed from below 170° C. to above 225° C. issufficient to provide a product solution having a mass ratio of totalpropylene glycol and ethylene glycol to said alkitol greater than about10:1.
 2. The process of claim 1 wherein the feed comprisesketose-yielding and aldose-yielding carbohydrate and a mixture ofpropylene glycol and ethylene glycol is contained in the productsolution.
 3. The process of claim 1 wherein the feed comprises sucrose.4. The process of claim 1 wherein the product solution contains a massratio of 1,2-butanediol to total propylene glycol and ethylene glycol ofless than about 1:10.
 5. The process of claim 1 wherein the aqueoussolution is maintained at a temperature of greater than about 170° C.and less than 230° C. for less than about 15 seconds prior to beingpassed into the aqueous-hydrogenation medium.
 6. The process of claim 1wherein the heating of the carbohydrate feed from below 170° C. to above230° C. is at least in part by direct heat exchange by admixing thecarbohydrate feed with a warmer fluid.
 7. The process of claim 6 whereinthe warmer fluid comprises the aqueous-hydrogenation medium.
 8. Theprocess of claim 6 wherein the admixing of the carbohydrate feed andwarmer fluid involves high shear mixing.
 9. The process of claim 6wherein the admixing of the carbohydrate feed and warmer fluid involvesrapid diffusional mixing.
 10. The process of claim 1 wherein the heatingof the carbohydrate feed from below 170° C. to above 230° C. is at leastin part by indirect heat exchange.
 14. The process of claim 1 whereinthe heating of the carbohydrate feed from below 170° C. to above 230° C.occurs whereupon the carbohydrate is contacted with an aqueous,retro-aldol solution containing retro-aldol catalyst in the substantialabsence of hydrogenation catalyst.
 15. The process of claim 1 whereinthe carbohydrate-containing feed contains between about 120 and 800grams of carbohydrate per liter of aqueous-hydrogenation medium.
 16. Theprocess of claim 1 wherein the carbohydrate-containing feed containsaldose and the rate of heating of the carbohydrate feed from below 170°C. to above 225° C. is sufficiently slow to provide a product solutionhaving provide a mass ratio of propylene glycol to ethylene glycol ofgreater that about 1:2.
 17. The process of claim 16 wherein the heatingof the carbohydrate feed from below 170° C. to above 225° C. isconducted in the presence of an isomerization catalyst or enzyme. 18.The process of claim 16 wherein the heating of the carbohydrate feedfrom below 170° C. to above 225° C. is conducted in the presence ofretro-aldol catalyst.
 19. The process of claim 1 wherein thecarbohydrate-containing feed contains at least about 50 mass percentketose-yielding carbohydrate based upon total carbohydrate in thecarbohydrate feed and the rate of heating of the carbohydrate feed from170° C. to above 225° C. is sufficiently rapid to provide a productsolution having a mass ratio of propylene glycol to ethylene glycol ofgreater that about 3:1.